Thin film transistor and display device

ABSTRACT

Provided is a thin film transistor capable of improving reliability in the thin film transistor including an oxide semiconductor layer. A thin film transistor including: a gate electrode; a gate insulating film formed on the gate electrode; an oxide semiconductor layer forming a channel region corresponding to the gate electrode on the gate insulating film; a channel protective film formed at least in a region corresponding to the channel region on the oxide semiconductor layer; and a source/drain electrode. A top face and a side face of the oxide semiconductor layer are covered with the source/drain electrode and the channel protective layer on the gate insulating film.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a thin film transistor (TFT) using anoxide semiconductor layer, and a display device including the same.

2. Description of the Related Art

An oxide semiconductor composed of zinc oxide, indium gallium zinc oxide(IGZO), or the like exhibits excellent characteristics as an activelayer of a semiconductor device, and development of the oxidesemiconductor has been in progress toward application to a TFT, a lightemitting device, a transparent conductive film, and the like in recentyears.

For example, the TFT using the oxide semiconductor has high electronmobility, and excellent electric characteristics in comparison with theTFT using amorphous silicon (a-Si:H) as a channel, which has been usedin the existing liquid crystal display device. Moreover, the TFT usingthe oxide semiconductor has an advantage that high mobility may beexpected even at a low temperature around a room temperature.

Meanwhile, it is known that the heat resistance of the oxidesemiconductor is not sufficient, and oxygen, zinc, or the like isdetached in heating treatment during manufacture process of the TFT andthe lattice defect is formed. The lattice defect results in formation ofan electrically-shallow impurity level, and causes low resistance of theoxide semiconductor layer. Thus, the operation of the oxidesemiconductor becomes normally-on type, that is, depression type inwhich a drain current flows without applying a gate voltage. The defectlevel is increased, the threshold voltage is reduced, and the leakagecurrent is increased.

By such lattice defect, induction of a carrier in zinc oxide to becomethe active layer is inhibited, and the carrier concentration is reduced.The reduction of the carrier concentration causes a reduction ofelectric conductivity of the active layer, and has an influence onelectron mobility and current transfer characteristics (for example,subthreshold characteristics and the threshold voltage) of the thin filmtransistor.

Thus, in the past, for example, it has been proposed that a gateinsulating layer in contact with a channel layer of oxide semiconductoris composed of amorphous aluminum oxide (Al₂O₃), and the defect level ofan interface is reduced (for example, Japanese Publication PatentNo.3913756).

SUMMARY OF THE INVENTION

Here, an oxide semiconductor film has characteristics that the carrierconcentration in the semiconductor is highly changed by absorption ofmoisture.

Thus, to realize the thin film transistor using the oxide semiconductorfilm as a channel, it is necessary to realize a device structure inwhich moisture mixing is suppressed. That is, it is desirable to improvereliability by suppressing absorption of moisture in the oxidesemiconductor layer.

In view of foregoing, it is desirable to provide a thin film transistorcapable of improving reliability in the thin film transistor includingan oxide semiconductor layer, and a display device including the same.

According to an embodiment of the present invention, there is provided athin film transistor including: a gate electrode; a gate insulating filmformed on the gate electrode; an oxide semiconductor layer formingchannel region corresponding to the gate electrode on the gateinsulating film; a channel protective film formed at least in a regioncorresponding to the channel region on the oxide semiconductor layer;and a source/drain electrode. A top face and a side face of the oxidesemiconductor layer are covered with the source/drain electrode and thechannel protective film on the gate insulating film.

According to an embodiment of the present invention, there is provided adisplay device including: a display element and a thin film transistordriving the display element and including a gate electrode, a gateinsulating film formed on the gate electrode, an oxide semiconductorlayer forming a channel region corresponding to the gate electrode onthe gate insulating film, a channel protective film formed at least in aregion corresponding to the channel region on the oxide semiconductorlayer, and a source/drain electrode. A top face and a side face of theoxide semiconductor layer are covered with the source/drain electrodeand the channel protective layer on the gate insulating film.

In the thin film transistor and the display device according to theembodiments of the present invention, the top face and the side face ofthe oxide semiconductor layer are covered with the source/drainelectrode and the channel protective layer on the gate insulating film.Thus, the oxide semiconductor layer is shut off from the external air,and mixing of moisture or the like to the oxide semiconductor layer issuppressed.

According to the thin film transistor and the display device of theembodiments of the present invention, the top face and the side face ofthe oxide semiconductor layer are covered with the source/drainelectrode and the channel protective layer on the gate insulating film.Thereby, mixing of moisture or the like to the oxide semiconductor layeris suppressed, and absorption of the moisture in the oxide semiconductorlayer is suppressed. Therefore, it is possible to improve reliability inthe thin film transistor including the oxide semiconductor layer.

Other and further objects, features and advantages of the invention willappear more fully from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating the configuration of a displaydevice according to an embodiment of the present invention.

FIG. 2 is an equivalent circuit view illustrating an example of a pixelcircuit illustrated in FIG. 1.

FIG. 3 is a plan view illustrating the configuration of a TFTillustrated in FIG. 2.

FIGS. 4A and 4B are cross sectional views illustrating the configurationof the TFT illustrated in FIG. 3.

FIG. 5 is a view illustrating a characteristic example of the TFT usingan oxide semiconductor.

FIG. 6 is a cross sectional view illustrating the detailed configurationof a source/drain electrode illustrated in FIGS. 3, 4A and 4B.

FIG. 7 is a cross sectional view illustrating a configuration example ofa display region illustrated in FIG. 1.

FIGS. 8A to 8C are cross sectional views illustrating an example of amethod of forming a TFT substrate (TFT) illustrated in FIGS. 3, 4A, and4B in step order.

FIGS. 9A and 9B are cross sectional views illustrating a step subsequentto FIGS. 8A to 8C.

FIG. 10 is a view for explaining influence of detachment of oxygen inthe oxide semiconductor on TFT operation.

FIG. 11 is a cross sectional view illustrating the configuration of aTFT according to a comparative example.

FIGS. 12A and 12B are characteristic views illustrating an example ofcurrent-voltage characteristics in the TFT of the comparative exampleand the TFT of the embodiment.

FIG. 13 is a cross sectional view illustrating the configuration of theTFT according to a modification of the present invention.

FIG. 14 is a plan view illustrating the schematic configuration of amodule including the display device of the embodiment.

FIG. 15 is a perspective view illustrating an appearance of a firstapplication example of the display device of the embodiment.

FIG. 16A is a perspective view illustrating an appearance as viewed froma front side of a second application example, and FIG. 16B is aperspective view illustrating an appearance as viewed from a rear sideof the second application example.

FIG. 17 is a perspective view illustrating an appearance of a thirdapplication example.

FIG. 18 is a perspective view illustrating an appearance of a fourthapplication example 4.

FIG. 19A is an elevation view of a fifth application example unclosed,FIG. 19B is a side view thereof, FIG. 19C is an elevation view of thefifth application example closed, FIG. 19D is a left side view thereof,FIG. 19E is a right side view thereof, FIG. 19F is a top face viewthereof, and FIG. 19G is a bottom view thereof.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment of the present invention will be hereinafter described indetail with reference to the drawings. The description will be made inthe following order:

-   1. Embodiment (example where a top face and a side face of an oxide    semiconductor layer is covered with a source/drain electrode and a    protective film)-   2. Modification (example where a protective film (passivation film)    is provided in addition to a channel protective layer)-   3. Module and application examples    1. EMBODIMENT

(Configuration Example of Display Device)

FIG. 1 illustrates the configuration of a display device according tothe embodiment of the present invention. This display device is used asan ultrathin organic light emitting color display device or the like. Inthe display device, for example, a display region 110 in which pixelsPXLCs composed of a plurality of organic light emitting elements 10R,10G, and 10B are arranged in matrix as a display element is formed in aTFT substrate 1. In the periphery of the display region 110, ahorizontal selector (HSEL) 121 as a signal section, a light scanner(WSCN) 131 as a scanner section, and a power source scanner (DSCN) 132are formed.

In the display region 110, signal lines DTL 101 to DTL 10 n are arrangedin the column direction, and scanning lines WSL 101 to WSL 10 m andpower source lines DSL 101 to DSL 10 m are arranged in the rowdirection. At each intersection of each signal line DTL and eachscanning line WSL, a pixel circuit 140 including the organic lightemitting element PXLC (one of the organic light emitting elements 10R,10G, and 10B (sub-pixel)) is provided. Each signal line DTL is connectedto the horizontal selector 121, and a video signal is supplied from thehorizontal selector 121 to the signal line DTL. Each scanning line WSLis connected to the light scanner 131. Each power source line DSL isconnected to the power source scanner 132.

FIG. 2 illustrates an example of the pixel circuit 140. The pixelcircuit 140 is an active drive circuit including a sampling transistor3A and a drive transistor 3B, a retention capacity 3C, and a lightemitting element 3D composed of the organic light emitting elementsPXLC. In the sampling transistor 3A, a gate is connected to thecorresponding scanning line WSL 101, one of a source and a drain isconnected to the corresponding signal line DTL 101, and the other of thesource and the drain is connected to a corresponding gate “g” of thedrive transistor 3B. In the drive transistor 3B, a drain “d” isconnected to the corresponding power source line DSL 101, and a source“s” is connected to an anode of the light emitting element 3D. A cathodeof the light emitting element 3D is connected to a ground wiring 3H. Theground wiring 3H is connected in common to all of the pixels PXLCs. Theretention capacity 3C is connected between the source “s” and the gate“g” of the drive transistor 3B.

The sampling transistor 3A is rendered conductive in response to acontrol signal supplied from the scanning line WSL 101, and samples asignal potential of the video signal supplied from the signal line DTL101. The sampled signal potential is retained in the retention capacity3C. The drive transistor 3B receives supply of a current from the powersource line DSL 101 in a first potential, and supplies a drive currentto the light emitting element 3D according to the signal potentialretained in the retention capacity 3C. The light emitting element 3Demits light at luminance according to the signal potential of the videosignal with the supplied drive current.

(Configuration Example of TFT)

FIG. 3 illustrates a plan configuration of a part of the pixel circuit140 in the TFT substrate 1 (part corresponding to the samplingtransistor 3A of FIG. 2). In the TFT substrate 1, for example, a TFT 20and the like composing the above-described sampling transistor 3A isformed on a substrate 10 made of glass or the like. Although it isomitted in FIG. 3, the drive transistor 3B of FIG. 2 has theconfiguration similar to that of the TFT 20.

FIGS. 4A and 4B illustrate the cross sectional structure of the TFT 20illustrated in FIG. 3. FIG. 4A illustrates the cross sectional structurealong line II-II of FIG. 3, and FIG. 4B illustrates the cross sectionalstructure along line III-III of FIG. 3, respectively.

The TFT 20 is, for example, a bottom gate type oxide semiconductortransistor including a gate electrode 21, a gate insulating film 22, anoxide semiconductor layer 23, a channel protective film 24, and asource/drain electrode 25 in this order on the substrate 10. Here,“oxide semiconductor” means oxide of zinc, indium, gallium, tin, ormixture of these, and it is known that the oxide semiconductor exhibitsexcellent semiconductor characteristics.

FIG. 5 illustrates current-voltage characteristics of an oxidesemiconductor TFT composed of, for example, mixed oxide of zinc, indium,and gallium (indium gallium zinc oxide: IGZO). The oxide semiconductorindicates electron mobility 10 times to 100 times higher than that ofamorphous silicon which has been used as the existing semiconductor, andexhibits favorable off characteristics. Moreover, the oxidesemiconductor indicates resistance 1/10 to 1/100 of that of the existingamorphous silicon, and the threshold voltage may be easily set low, forexample, 0 V or less.

The gate electrode 21 controls electron density in the oxidesemiconductor layer 23 with a gate voltage applied to the TFT 20. Thegate electrode 21 has, for example, a two-layer structure including amolybdenum (Mo) layer having a thickness of 50 nm, and an aluminum (Al)layer or an aluminum alloy layer having a thickness of 400 nm.

The gate insulating film 22 has, for example, a two-layer structureincluding a gate insulating film 221 having a thickness of approximately380 nm, and a gate insulating film 222 having a thickness ofapproximately 20 nm in this order from the substrate 10 side. Such gateinsulating films 221 and 222 are composed of a single layer film of asilicon oxide film, a silicon nitride film, a silicon nitride oxidefilm, an aluminum oxide film, or the like, or a stacked layer film ofthese. Among them, the aluminum oxide film and the silicon nitride filmlayer which exhibit high barrier characteristics to moisture and thelike are preferably used for the gate insulating films 221 and 222. Inthat case, it is possible to suppress moisture diffusion from thesubstrate 10 side.

The oxide semiconductor layer 23 has a thickness of, for example, 50 nm,and is composed of indium gallium zinc oxide (IGZO). In the oxidesemiconductor layer 23, a channel region (not illustrated in the figure)is formed corresponding to the gate electrode 21. The oxidesemiconductor layer 23 is patterned into an island shape (notillustrated in the figure).

Here, as illustrated in FIGS. 4A and 4B, on the gate insulating film222, the top face and the side face of the oxide semiconductor layer 23are covered with the source/drain electrode 25 and the channelprotective film 24 which will be described later. Thereby, although thedetail will be described later, even when the existing passivation film(protective film) is not formed, the TFT 20 may be stably operatedwithout being influenced by moisture or the like in the air.

The channel protective film 24 is formed at least in a regioncorresponding to the channel region in the oxide semiconductor layer 23,and has a three-layer structure including channel protective films 241,242, and 243 in this order from the substrate 10 side. These channelprotective films 241 to 243 contain aluminum oxide (Al₂O₃) or siliconnitride (SiN or the like), and are specifically composed of an aluminumoxide film, a silicon nitride film, or a silicon oxynitride film.However, among these, at least one layer of the channel protective films242 and 243 is composed of the aluminum oxide film or the siliconnitride film. The channel protective film 24 having such a structureprevents damage of the channel region in the oxide semiconductor layer23, and prevents infiltration of hydrogen, moisture, or the like to theoxide semiconductor layer 23. The channel protective film 24 alsoprotects the channel region from resist stripping liquid or the likeused at the time of forming the source/drain electrode 25.

The source/drain electrode 25 is composed of, for example, a multilayerfilm in which a plurality of metal layers are stacked. Specifically, forexample, as illustrated in FIG. 6, the source/drain electrode 25 has,for example, the stacked structure of three layers including a firstmetal layer 251 having a thickness of approximately 50 nm, a secondmetal layer 252 having a thickness of approximately 500 nm, and a thirdmetal layer 253 having a thickness of approximately 50 nm. Among them,the first metal layer 251 is formed along an interface with the oxidesemiconductor layer 23, and the second metal layer 252 and the thirdmetal layer 253 are formed in this order on the first metal layer 251.Such metal layers 251 to 253 are composed of a metal material such asmolybdenum (Mo), aluminum (Al), copper (Cu), titanium (Ti), ITO(indium-tin composite oxide), or titanium oxide (TiO), respectively.However, among the metal layers 251 to 253, the first metal layer 251 incontact with the oxide semiconductor layer 23 is preferably composed ofa metal material containing molybdenum (Mo) or oxygen (such as ITO andtitanium oxide). In this case, it is possible to stabilize electriccharacteristics of the TFT 20 while suppressing the oxygen detachment inthe oxide semiconductor layer 23.

(Example of Cross Sectional Configuration of Display Region)

FIG. 7 illustrates the cross sectional configuration of the displayregion 110 illustrated in FIG. 1. In the display region 110, the organiclight emitting element 10R emitting red light, the organic lightemitting element 100 emitting green light, and the organic lightemitting element 10B emitting blue light are formed in this order inmatrix as a whole. The organic light emitting elements 10R, 10G, and 10Bhave a strip planar shape, and a combination of the organic lightemitting elements 10R, 10G, and 10B adjacent to each other compose onepixel.

The organic light emitting elements 10R, 10G, and 10B have theconfiguration in which an anode 52, an interelectrode insulating film54, an organic layer 53 including a light emitting layer which will bedescribed later, and a cathode 55 are stacked in this order on the TFTsubstrate 1 with a planarizing insulating film 51 in between,respectively.

Such organic light emitting elements 10R, 10G, and 10B are covered witha protective film 56 made of silicon nitride (SiN), silicon oxide (SiO),or the like, if necessary. The organic light emitting elements 10R, 10G,and 10B are sealed by adhering a sealing substrate 71 made of glass orthe like onto the whole surface of the protective film 56 with anadhesion layer 60 made of thermal curing resin, ultraviolet curingresin, or the like in between. The sealing substrate 71 may be providedwith a color filter 72 and a light blocking film (not illustrated in thefigure) as black matrix, if necessary.

The planarizing insulating film 51 is intended to planarize the surfaceof the TFT substrate 1 in which the pixel circuit 140 is formed, and ispreferably composed of a material having a good pattern accuracy to forma fine connecting hole 51A. As the material for the planarizinginsulating film 51, for example, there is an organic material such aspolyimide, or an inorganic material such as silicon oxide (SiO₂). Thedrive transistor 3B illustrated in FIG. 2 is electrically connected tothe anode 52 through the connecting hole 51 A provided in theplanarizing insulating film 51. Although it is omitted in FIG. 7, alower layer electrode 31 of the capacitor 30 composing the retentioncapacity 3C is electrically connected to the anode 52 through aconnecting hole (not illustrated in the figure) provided in theplanarizing insulating film 51 (refer to FIG. 2).

The anode 52 is formed corresponding to each of the organic lightemitting elements 10R, 10G, and 10B. The anode 52 serves as a reflectiveelectrode reflecting light generated in the light emitting layer, andpreferably has reflectance as high as possible to increase the lightemission efficiency. The anode 52 has a thickness of, for example, 100nm to 1000 nm both inclusive, and is composed of a single metal elementof silver (Ag), aluminum (Al), chrome (Cr), titanium (Ti), iron (Fe),cobalt (Co), nickel (Ni), molybdenum (Mo), copper (Cu), tantalum (Ta),tungsten (W), platinum (Pt), gold (Au), or the like, or an alloy ofthese.

The interelectrode insulating film 54 is intended to ensure insulationbetween the anode 52 and the cathode 55 and intended to accurately shapea light emitting region into a desirable shape. The interelectrodeinsulating film 54 is composed of, for example, an organic material suchas polyimide, or an inorganic insulating material such as silicon oxide(SiO₂). The interelectrode insulating film 54 has an aperturecorresponding to the light emitting region in the anode 52. Although theorganic layer 53 and the cathode 55 may be provided not only on thelight emitting region but also on the interelectrode insulating film 54continuously, light is emitted only in the aperture of theinterelectrode insulating film 54.

The organic layer 53 has the configuration, for example, in which a holeinjection layer, a hole transport layer, a light emitting layer, and anelectron transport layer (all not illustrated in the figure) are stackedin this order from the anode 52 side. Among them, the layers other thanthe light emitting layer may be provided, if necessary. The organiclayer 53 may have the different configuration depending on the lightemission color of the organic light emitting elements 10R, 10G, and 10B.The hole injection layer is intended to increase the hole injectionefficiency, and a buffer layer to prevent leakage. The hole transportlayer is intended to increase the hole transport efficiency to the lightemitting layer. Recombination of an electron and a hole occurs byapplying an electric field, and the light emitting layer generateslight. The electron transport layer is intended to increase the electrontransport efficiency to the light emitting layer. The material for theorganic layer 53 is not specifically limited, as long as it is a typicallow polymer organic material or a typical high polymer organic material.

The cathode 55 has a thickness of, for example, 5 nm to 50 nm bothinclusive, and is composed of a single metal element of aluminum (Al),magnesium (Mg), calcium (Ca), or sodium (Na), or an alloy of these.Among them, an alloy of magnesium and silver (MgAg alloy), or an alloyof aluminum (Al) and lithium (Li) (AlLi alloy) is preferable. Thecathode 55 may be composed of ITO (indium-tin composite oxide), or IZO(indium-zinc composite oxide).

(Example of Method of Manufacturing Display Device)

The display device may be manufactured, for example, in the followingmanner.

(Step of Forming TFT 20)

FIGS. 8A to 8C, and 9 illustrate cross sectional views of an example ofa step of forming the TFT substrate 1 including the TFT 20.

As illustrated in FIG. 8A, for example, a two-layer structure includinga molybdenum (Mo) layer having a thickness of 50 nm, and an aluminum(Al) layer or an aluminum alloy layer having a thickness of 400 nm isformed on the substrate 10 made of glass through the use of, forexample, sputtering method. Next, photolithography and etching areperformed on the two-layer structure, thereby forming the gate electrode21.

Next, a two-layer structure including a silicon nitride film to becomethe gate insulating film 221, and a silicon oxide film to become thegate insulating film 222 are formed on the whole surface of thesubstrate 10 through the use of, for example, sputtering method orplasma CVD (chemical vapor deposition) method. At this time, to givepriority to productivity, the whole gate insulating film 22 may be asingle layer film of an aluminum oxide film, a silicon oxide film, asilicon oxynitride film, or a silicon nitride film. Since the siliconoxide film or the silicon oxynitride film formed on the silicon nitridefilm is removed by etching immediately after processing the oxidesemiconductor layer 23 into the island shape, it is desirable to formthe silicon oxide film or the silicon oxynitride film in a filmthickness of 5 nm to 50 nm both inclusive.

After that, as illustrated in FIG. 8B, an indium gallium zinc oxide(IGZO) film to become the oxide semiconductor layer 23 is formed in athickness of approximately 5 nm to 100 nm both inclusive through the useof, for example, sputtering method. At that time, for example, in thecase where the oxide semiconductor layer 23 is composed of IGZO, DCsputtering method targeting ceramic of IGZO is used, and the oxidesemiconductor layer 23 is formed on the substrate 10 through the use ofplasma discharge by mixed gas of argon (Ar) and oxygen (O₂). Before theplasma discharge, the air in a vacuum container is exhausted until thevacuum level of inside the vacuum container becomes 1×10⁻⁴ Pa or less,and then the mixed gas of argon and oxygen is introduced. For example,in the case where the oxide semiconductor layer 23 is composed of zincoxide, the oxide semiconductor layer 23 is formed through the use of RFsputtering method targeting ceramic of zinc oxide, or DC sputteringmethod in a gas atmosphere containing argon and oxygen by using a metaltarget of zinc.

After forming the oxide semiconductor layer 23, an aluminum oxide film,a silicon oxide film, or a silicon oxynitride film to become the channelprotective film 241 is formed in a thickness of, for example, 10 nm to50 nm both inclusive through the use of, for example, sputtering methodor CVD method. At this time, in the case where the aluminum oxide filmor the silicon oxide film is formed through the use of sputteringmethod, it is preferable to continuously form the oxide semiconductorlayer 23 and the aluminum oxide film or the like in a sputtering device.Thereby, it is possible to uniform the characteristics of the TFT 20. Inthe case where priority is given to the productivity, it may proceed toa next step without forming such a channel protective film 241.

Next, as illustrated in FIG. 8C, the oxide semiconductor layer 23 tobecome the channel region of the TFT 20, and the channel protective film241 are formed and patterned into the island shape.

Next, as illustrated in FIG. 9A, an oxide aluminum film, a silicon oxidefilm, or a silicon oxynitride film to become the channel protectivefilms 242 and 243 is formed in a thickness of 100 nm through the use of,for example, CVD method. At this time, for example, the channelprotective film 242 may be composed of an aluminum oxide film, and thechannel protective film 243 may be composed of a silicon oxide film orsilicon oxynitride film. Reversely, the channel protective film 242 maybe composed of a silicon oxide film or a silicon oxynitride film, andthe channel protective film 243 may be composed of an aluminum oxidefilm. In the case where the silicon oxide film or the silicon oxynitridefilm is formed through the use of plasma CVD method or the like, it ispossible to easily increase the film thickness of the whole channelprotective film 24 to approximately 200 nm which is the necessarythickness in etching for forming the source/drain electrode 25.

Next, as illustrated in FIG. 9B, photolithography and etching areperformed on the aluminum oxide film, the silicon oxide film, or thesilicon oxynitride film, thereby shaping the aluminum oxide film, thesilicon oxide film, or the silicon oxynitride film into a predeterminedshape. Thereby, the channel protective film 24 including the channelprotective films 241 to 243 having the shape illustrated in FIG. 4 isformed.

Here, there is an issue that the electric characteristics of thesemiconductor in the oxide semiconductor layer 23 are changed byabsorption of moisture or the like. Thus, by using the aluminum oxidefilm as the channel protective film, it is possible to stabilize theelectric characteristics of the TFT 20 with excellent gas barriercharacteristics of the aluminum oxide film, and it is possible to formthe channel protective film without deteriorating the characteristics ofthe TFT 20. In the case where such an aluminum oxide film is formedthrough the use of atomic layer deposition (ALD) method, it is possibleto form a dense insulating film in the condition where generation ofhydrogen deteriorating the electric characteristics of the oxidesemiconductor layer 23 is suppressed. Here, in the case where the atomiclayer deposition method is used, trimethyl aluminum gas as a rawmaterial gas is introduced into a vacuum chamber, and an aluminum filmof an atomic layer is formed on the surface of the substrate 10. Next,an oxygen radical in which an ozone gas or an oxygen gas is excited withplasma is introduced to the surface of the substrate 10, and thealuminum film is oxidized. Since the aluminum film formed first has athickness of the atomic layer, it is possible to easily oxidize thealuminum film with the ozone or the oxygen radical, and it is possibleto form the aluminum oxide film which is uniform on the whole substrate10. After that, by repeating formation of the aluminum film and theoxidizing process, it is possible to form the channel protective filmcomposed of the aluminum oxide film having a predetermined thickness. Inthis method, the aluminum oxide film may have a composition whichcorresponds to the stoichiometric ratio without shortage of the oxygenconcentration in the aluminum oxide film. Thus, the composition ratio ofaluminum and oxygen may be ideal as 2:3, and the channel protective filmhaving the excellent electric characteristics and the excellent gasbarrier characteristics may be formed.

Next, the source/drain electrode 25 is formed on the gate insulatingfilm 222, the oxide semiconductor layer 23, and the channel protectivefilm 24 through the use of, for example, sputtering method.Specifically, for example, a molybdenum layer (first metal layer 251)having a thickness of approximately 50 nm, an aluminum layer (secondmetal layer 252) having a thickness of approximately 500 nm, and amolybdenum layer (third metal layer 253) having a thickness of 50 nm areformed in this order. The first metal layer 251 to the third metal layer253 are shaped into a predetermined shape through the use ofphotolithography and etching (for example, wet etching method usingmixed liquid containing phosphoric acid, nitric acid, and acetic acid),respectively. Thereby, the source/drain electrode 25 is formed. Asdescribed above, the TFT substrate 1 including the TFT 20 as illustratedin FIGS. 3, 4A and 4B is formed.

(Step of Forming Organic Light Emitting Elements 10R, 10G, and 10B)

First, photosensitive resin is applied over the whole surface of the TFTsubstrate 1, and exposure and development are performed on the TFTsubstrate 1. Thereby, the planarizing insulating film 51 and theconnecting hole 51A are formed and burned. Next, the anode 52 made ofthe above-described material is deposited through the use of, forexample, direct sputtering, and is selectively etched through the useof, for example, lithography technique to pattern the anode 52 into apredetermined shape. Next, the interelectrode insulating film 54 havingthe above-described thickness and made of the above-described materialis formed through the use of, for example, CVD method, and the apertureis formed through the use of, for example, lithography method. Afterthat, the organic layer 53 and the cathode 55 made of theabove-described material are deposited in this order through the use of,for example, evaporation method, and the organic light emitting elements10R, 10G, and 10B are formed. Next, the organic light emitting elements10R, 10G, and 10B are covered with the protective film 56 made of theabove-described material.

After that, the adhesion layer 60 is formed on the protective film 56.After that, the color filter 72 is provided, and the sealing substrate71 made of the above-described material is prepared. The TFT substrate 1and the sealing substrate 71 are adhered to each other with the adhesionlayer 60 in between. As described above, the display region illustratedin FIG. 7 is completed.

(Action and Effect of Display Device)

Next, the description will be made on action and effect of the displaydevice of this embodiment by comparing with a comparative example.

In the display device of this embodiment, the sampling transistor 3A isrendered conductive in response to the control signal supplied from thescanning line WSL, and the signal potential of the video signal suppliedfrom the signal line DTL is sampled and retained in the retentioncapacity 3C. The current is supplied from the power source line DSL 101in the first potential to the drive transistor 3B, and the drive currentis supplied to the light emitting element 3D (organic light emittingelements 10R, 10G, and 10B) according to the signal potential retainedin the retention capacity 3C. The light emitting element 3D (organiclight emitting elements 10R, 10G, and 10B) emits light at luminanceaccording to the signal potential of the video signal with the supplieddrive current. This light transmits the cathode 55, the color filter 72,and the sealing substrate 71, and is extracted.

Here, the heat resistance of the oxide semiconductor is not sufficient,and the oxygen is detached in the heating treatment and the plasmaprocess during the manufacture process of the TFT and the lattice defectis formed. The lattice defect results in formation of theelectrically-shallow impurity level, and causes low resistance of theoxide semiconductor. Thus, in the case where the oxide semiconductor isused as the active layer of the TFT, the defect level is increased, thethreshold voltage is reduced, and the leakage current is increased,resulting that the operation of the oxide semiconductor becomesdepression type in which the drain current flows without applying thegate voltage. When the defect level is sufficiently increased, forexample, as illustrated in FIG. 10, the transistor operation is stoppedto shift to the conductor operation.

Comparative Example

Thus, in a TFT 820 according to the comparative example whose crosssectional structure is illustrated in FIG. 11, on the gate insulatingfilm 22, the top face of the oxide semiconductor layer 23 is coveredwith the source/drain electrode 25 and the channel protective film 24.However, in the TFT 820, unlike the TFT 20 of this embodiment, the sideface of the oxide semiconductor layer 23 is not covered, and is exposedto the external air (refer to regions of reference numerals P801 andP802 in the figure). Therefore, in the TFT 820, since moisture or thelike in the external air is supplied into the oxide semiconductor layer23, the electric characteristics of the TFT 820 become unstable.

(This Embodiment)

On the other hand, in this embodiment, on the gate insulating film 22,the top face and the side face of the oxide semiconductor layer 23 arecovered with the source/drain electrode 25 and the channel protectivefilm 24. Thereby, unlike the comparative example, the oxidesemiconductor 23 is shut off from the external air, and mixing ofmoisture or the like to the oxide semiconductor layer 23 is suppressed.Therefore, in the TFT 20 of this embodiment, the electriccharacteristics are stabilized in comparison with those of the TFT 820of the comparative example.

FIGS. 12A and 12B illustrate the change of the characteristics(current-voltage characteristics) indicating the relationship betweenthe gate voltage and the drain current of the thin film transistor, inthe case where the thin film transistor is set in a high-temperaturehigh-humidity furnace under an environment of temperature of 60° C. andhumidity of 90% for 24 hours. Here, FIG. 12A illustrates thecurrent-voltage characteristics of the TFT 820 according to thecomparative example, and FIG. 12B illustrates the current-voltagecharacteristics of the TFT 20 of this embodiment, respectively. It canbe seen that the current-voltage characteristics are hardly changed inthe TFT 20 of this embodiment illustrated in FIG. 12B, while thethreshold voltage is changed in the minus direction in the TFT 820 ofthe comparative example illustrated in FIG. 12A.

As described above, in this embodiment, since the top face and the sideface of the oxide semiconductor layer 23 are covered with thesource/drain electrode 25 and the channel protective film 24 on the gateinsulating film 22, mixing of moisture or the like to the oxidesemiconductor layer 23 is suppressed, and absorption of the moisture inthe oxide semiconductor layer 23 is suppressed. Therefore, it ispossible to improve reliability in the thin film transistor includingthe oxide semiconductor layer.

In the display device using such a TFT 20, it is possible to realize aninexpensive high-quality flat panel display.

2. MODIFICATION

FIG. 13 illustrates the cross sectional structure of a TFT 20A accordingto a modification of the present invention. In the figure, samereference numerals as in the above embodiment have been used to indicatesubstantially identical components, and thereby the description isappropriately omitted.

In the TFT 20A, in addition to the hitherto-described channel protectivefilm 24, a protective film (passivation film) 26 is provided so as tocover the whole outermost surface of the TFT 20A. In the same manner asthe channel protective film 24, such a protective film 26 preferablycontains aluminum oxide or silicon nitride, and may be a stacked filmincluding at least one layer of the aluminum oxide film and the siliconnitride film.

In the case of such a configuration, the TFT 20A may be strictlyprotected from the external moisture or the like, and it is possible toimprove the reliability of the thin film transistor more.

3. Module And Application Example

Hereinafter, application examples of the display device which has beendescribed in the above embodiment and the modification will bedescribed. The display device of the above embodiment and the like maybe applied to electric devices of various fields such as a televisiondevice, a digital camera, a notebook personal computer, a mobileterminal device such as a mobile phone, or a video camera. In otherwords, the display device of the above embodiment and the like may beapplied to display device in electric devices of various fields in whicha video signal input from the external or a video signal generatedinside the device is displayed as an image or a video.

(Module)

The display device of the above embodiment is, for example, installed asa module illustrated in FIG. 15 in various electric devices of a firstapplication example to a fifth application example which will bedescribed later. In this module, for example, an exposed region 210exposed from the sealing substrate 71 and the adhesion layer 60 isprovided on one side of a substrate 11, and an external connectingterminal (not illustrated in the figure) is formed by extending wiringof a signal line drive circuit 120 and a scanning line drive circuit 130in the exposed region 210. In the external connecting terminal, aflexible printed circuit (FPC) 220 may be provided for input/output of asignal.

First Application Example

FIG. 15 illustrates an appearance of a television device to which thedisplay device of the above embodiment and the like is applied. Thetelevision device includes, for example, a video display screen section300 including a front panel 310 and a filter glass 320. The videodisplay screen section 300 is composed of the display device accordingto the above embodiment and the like.

Second Application Example

FIGS. 16A and 16B illustrate an appearance of a digital camera to whichthe display device of the above embodiment and the like is applied. Thedigital camera includes, for example, a light emitting section for aflash 410, a display section 420, a menu switch 430, and a shutterbutton 440. The display section 420 is composed of the display deviceaccording to the above embodiment and the like.

Third Application Example

FIG. 17 illustrates an appearance of a notebook personal computer towhich the display device of the above embodiment and the like isapplied. The notebook personal computer includes, for example, a mainbody 510, a keyboard 520 for operation of inputting characters and thelike, and a display section 530 for displaying an image. The displaysection 530 is composed of the display device according to the aboveembodiment and the like.

Fourth Application Example

FIG. 18 illustrates an appearance of a video camera to which the displaydevice of the above embodiment and the like is applied. The video cameraincludes, for example, a main body 610, a lens for capturing an object620 provided on the front side face of the main body 610, a start/stopswitch in capturing 630, and a display section 640. The display section640 is composed of the display device according to the above embodimentand the like.

Fifth Application Example

FIGS. 19A to 19G illustrate an appearance of a mobile phone to which thedisplay device of the above embodiment and the like is applied. In themobile phone, for example, an upper package 710 and a lower package 720are jointed by a joint section (hinge section) 730. The mobile phoneincludes a display 740, a sub-display 750, a picture light 760, and acamera 770. The display 740 or the sub-display 750 is composed of thedisplay device according to the above embodiment and the like.

Hereinbefore, although the present invention has been described withreference to the embodiment, the modification, and the applicationexamples thereof, the present invention is not limited to the embodimentand the like, and various modifications may be made.

For example, the material, the thickness, the film-forming method, thefilm-forming conditions and the like of each layer are not limited tothose described in the above embodiment. Other material, otherthickness, other film-forming method, and other film-forming conditionsmay be adopted.

In the embodiment and the like, the description has been specificallymade by using the configuration of the organic light emitting elements10R, 10B, and 10G. However, it is not always necessary to include allthe layers, and other layers may be additionally included.

In addition, the present invention may be applied to a display deviceusing other display element such as a liquid crystal display element, aninorganic electroluminescence element, an electrodeposition type displayelement, and an electrochromic type display element in addition to theorganic EL display element.

The present application contains subject matter related to thatdisclosed in Japanese Priority Patent Application JP 2009-24035 filed inthe Japan Patent Office on Feb. 4, 2009, the entire content of which ishereby incorporated by reference.

It should be understood by those skilled in the art that variousmodifications, combinations, sub-combinations and alternations may occurdepending on design requirements and other factors insofar as they arewithin the scope of the appended claims or the equivalents thereof.

What is claimed is:
 1. A thin film transistor comprising: a gateinsulating film between an oxide semiconductor layer and a gateelectrode, said gate electrode being between said gate insulating filmand a substrate; a source/drain electrode in physical contact with asource/drain region of the oxide semiconductor layer and said gateinsulating film, said source/drain region being between said gateinsulating film and said source/drain electrode; a channel protectivefilm between said source/drain electrode and a different source/drainelectrode, a channel region of the oxide semiconductor layer beingbetween said gate insulating film and said channel protective film,wherein said channel protective film comprises a second protective filmbetween a first protective film and a third protective film, said secondprotective film being in physical contact with said first protectivefilm and said third protective film, wherein said first protective filmis in physical contact with said oxide semiconductor layer, said secondprotective film being in physical contact with said gate insulatingfilm, wherein along every direction within a plan view of a layout, aboundary of the source/drain electrode extends beyond a boundary of thesource/drain region.
 2. The thin film transistor according to claim 1,wherein said channel protective film is in physical contact with saidgate insulating film and said oxide semiconductor.
 3. The thin filmtransistor according to claim 1, wherein said channel protective film isin physical contact with said source/drain electrode and said differentsource/drain electrode.
 4. The thin film transistor according to claim1, wherein said channel protective film is from the group consisting ofan aluminum oxide film, a silicon nitride film, and a silicon oxynitridefilm.
 5. The thin film transistor according to claim 1, wherein saidchannel region is between said source/drain region and a differentsource/drain region of the oxide semiconductor layer.
 6. The thin filmtransistor according to claim 5, wherein said different source/drainelectrode is in physical contact with said gate insulating film and saiddifferent source/drain region.
 7. The thin film transistor according toclaim 5, wherein along said every direction within the plan view of thelayout, a boundary of the different source/drain electrode extendsbeyond a boundary of the different source/drain region.
 8. The thin filmtransistor according to claim 1,wherein said second protective film isan aluminum oxide film or a silicon nitride film.
 9. The thin filmtransistor according to claim 1, wherein said third protective film isan aluminum oxide film or a silicon nitride film.
 10. The thin filmtransistor according to claim 1, wherein said gate insulating filmincludes a film from the group consisting of a silicon oxide film, asilicon nitride film, a silicon nitride oxide film, and an aluminumoxide film.
 11. The thin film transistor according to claim 1, whereinsaid gate insulating film includes a first insulating film and a secondinsulating film, said second insulating film being between said firstinsulating film and said oxide semiconductor layer.
 12. The thin filmtransistor according to claim 11, wherein said first insulating film isa silicon nitride film.
 13. The thin film transistor according to claim11, wherein said second insulating film is a silicon oxide film.
 14. Thethin film transistor according to claim 1, wherein said substrate is aglass substrate.
 15. The thin film transistor according to claim 1,wherein said oxide semiconductor layer includes an element from thegroup consisting of zinc, indium, gallium, and tin.
 16. The thin filmtransistor according to claim 1, wherein said source/drain electrodeincludes a metal material from the group consisting of molybdenum,aluminum, copper, titanium, indium-tin composite oxide, and titaniumoxide.
 17. The thin film transistor according to claim 1, wherein saidsource/drain electrode comprises a second metal layer between a firstmetal layer and a third metal layer, said second metal layer being inphysical contact with said first metal layer and said third metal layer.18. A display device comprising: the thin film transistor according toclaim 1; and a display element, the thin film transistor beingconfigured to drive said display element.